In the field of (meth)acrylic polymers, the radical polymerization of (meth)acrylic monomers in a solvent medium is one of the routes conventionally used. Polymerization is initiated using compounds generating free radicals such as organic peroxides or nitrous compounds which, depending on cases, may or may not be combined with decomposition accelerators as is the case in particular in oxidation-reducing initiator systems also known as “redox”. In radical polymerization, the length of the kinetic chain γ of the polymers is based on the expression: kp [M]
Y=2(f kd kt [I])1Ω where:
kp: rate constant of propagation; [M]: monomer concentration; f: initiator efficiency factor; kd: rate constant of initiator decomposition; kt: rate constant of termination; [I]: initiator concentration.
The weight average molecular weights of the polymers, obtained using this method, are relatively high, generally higher than 50,000 g/mole. However it may be necessary to provide polymers having a lower molecular weight, in particular for reasons of efficiency in some industrial applications or to meet environmental constraints as is the case in the area of coatings containing solvated acrylic reins. In this case, the use of low molecular weight resins effectively allows the dry extract of the solvated formulations to be increased without modifying the viscosity thereof, which helps towards reducing the content of volatile organic compounds.
Some applications of (meth)acrylic polymers also require that they should be free of impurities which may generate problems of odour or colour change such as sulphur-containing compounds, and that they should be free of metals or halogens. The synthesis of low molecular weight (meth)acrylic polymers in solvent medium raises known problems in the state of the art and numerous alternatives have already been developed for the purpose of limiting molecular weights.
For example the very high temperature thermal polymerization described for example in U.S. Pat. Nos. 6,844,406 and 4,117,235, allows a large number of radicals to be generated by accelerating the decomposition kinetics of the initiator whilst increasing the probable occurrence of transfer reactions. This gives polymers of lower molecular weight having reduced viscosity in solution. However, this process applying a temperature generally of between 150 and 250° C., sometimes higher than 300° C., is limited to the use of solvents with high boiling point and not only entails the use of high concentrations of initiator but also high energy costs.
In U.S. Pat. No. 4,652,605 relating to the preparation of vinyl polymers of low molecular weight, the control over the length of the kinetic chain is ensured by adding a high concentration of initiator. Polymerization under dilute reaction conditions also allows a reduction in the probable occurrence of propagation reactions and thereby contributes towards obtaining low molecular weights. Industrially, this method is not economic however on account of the high cost of the initiators generally used and the low polymer productivity thereof.
In U.S. Pat. No. 4,056,559, methacrylic esters of weight average molecular weight of between 400 and 10,000 are prepared by anionic polymerization in the presence of an alkoxide anion as catalyst and an alcohol which acts as chain regulating agent, the control over molecular weight being achieved through the ratio between the total quantity of alcohol used and the monomer charge. The catalyst, such as sodium methoxide can be neutralized after the reaction using a mineral acid such as hydrochloric acid, and a polar organic solvent such as dimethylsulphoxide can be used to improve the solubility and efficacy of the catalyst. It is therefore not excluded that the polymers thus obtained contain traces of halogen or sulphur compound.
Another route very widely used to limit the molecular weights of polymers obtained by radical polymerization is the use of a chain transfer agent. The role of this agent is to limit the lengths of polymer chains through the early terminating of the active growth centres. The chain transfer agent may be the monomer itself, as is the case for allylic monomers, or the solvent particularly if it is a protic solvent such as isopropanol used in the process described in document U.S. Pat. No. 4,301,266. Industrially, the most frequently used chain transfer agents are sulphur-containing agents such as alkyl mercaptans and in particular lauryl mercaptan (FR 2,604,712, U.S. Pat. No. 3,028,367). While their high transfer constants make these the compounds of choice for efficient control over molecular weights, these compounds have the disadvantage however of leading to odorous polymers liable to colour-change problems in storage. They are also incompatible with applications in which sulphur is prohibited. Other so-called controlled radical polymerization techniques also allow the molecular weights of the polymer chains to be limited. Among these mention may be made of nitroxide mediated polymerization. (NMP), polymerization initiated by species acting simultaneously as transfer agent, polymerization in the presence of organometallic compounds or polymerization processes by atom transfer (ATRP/RAFT) which have recourse to sulphur- or halogen-containing molecules. These processes have various shortcomings however such as difficult implementation, use of compounds possibly leading to the presence of undesirable impurities such as sulphur-containing, metallic or halogenated compounds, and a high cost.
Document U.S. Pat. No. 5,475,073 describes a method for preparing hydroxy-functionalized acrylic resins in which an allylic alcohol acts both as solvent and as transfer agent, and the initiator is partly added by continuous addition throughout polymerization. The resins have a number average molecular weight ranging from about 500 to about 10,000 g/mole.
The article Paint India 53 (8) (August 2003) by Asian PPG, pages 33-46, describes a novel route for synthesizing low molecular weight acrylic resins, based on adding the initiator to the reactor either in part or in full before adding the monomers. The monomers are a mixture of acrylic acid, methyl methacrylate, 2-ethyl hexyl acrylate, hydroxy ethyl methacrylate and styrene. The peroxide of di tert-butyl (DTBP) is used as initiator and o-xylene as solvent. The polymerization temperature is 132° C. The weight average molecular weights of the resins obtained are between 18,000 and 30,000 g/mole.
WO 2008/006998 describes (meth)acrylic polymers of low molecular weight ( Mw lower than 20,000 g/mole) which are prepared by radical polymerization in solution, the entirety of the radical initiator being added as starter to the reactor vessel before continuously adding the monomers in the presence of a decomposition accelerator. However, it has been ascertained that with this preparation method the level of residual monomers is high (more than 18% even up to 20%, hence one fifth of the monomers used which are not polymerized) which is detrimental for envisaged applications such as the use as additives in oil, hydrocarbon, lubricant formulations.
In WO 2008/006965 the use of a homopolymer is described in hydrocarbon distillates having a boiling point of between 150 and 450° C., the homopolymer being obtained from C3-C12 olefin esters of carboxylic acid and a fatty alcohol comprising a chain of more than 16 carbon atoms and optionally an olefin bond, for enhancing the efficiency of filterability additives added to the said distillates containing a co- and/or terpolymer of ethylene, C3-C5 carboxylic acid vinyl ester and a mono-alcohol comprising 1 to 10 carbon atoms. These homopolymers were prepared following the operating mode described in WO 2008/006968 (cf. page 13, lines 7-17).